The present invention concerns a pressure sensor of the semiconductor-on-insulator type, and a piezoresistive element suitable for incorporation in such a sensor.
Pressure sensors of the diffused gauge type are well known. European Patent Application No. 109992 describes such a sensor. A very thin deformable diaphragm is formed by machining in a semiconductor wafer, for example of silicon, and a border is left in existence around the diaphragm for mounting the diaphragm in the body of the sensor. To measure the pressure or the pressure difference applied to the diaphragm, piezoresistive gauges are formed on the diaphragm by localized doping of the semiconductor material. In general, there are four diffused gauges mounted in a Wheatstone bridge. One possible solution for implanting the gauges is the following: two of the gauges are disposed in zones of the diaphragm where the stresses due to the pressure are positive, the other two being disposed in zones where the stresses due to the pressure are negative.
One of the problems raised by diffused gauge sensors resides in the fact that it is very difficult to achieve a thorough electrical isolation between the gauges, since they are formed by P-type implantation or diffusion in a substrate of N-type silicon and the insulation between each gauge and the substrate is formed by a PN junction. This insulation problem is further increased when the temperature to which this sensor is subjected increases. Typically, these sensors are limited to a temperature of 130.degree. C.
The technology of silicon-on-insulator (SOI) permits the problem of insulation between the substrate and the gauges to be resolved. The article by E. Obermeier published in IEEE Transactions 1985, pages 430 to 433, describes such a pressure sensor. The silicon substrate is covered with an insulating layer, for example silicon oxide, and the piezoresistive gauges are individually formed on the insulating layer. As in the case of diffused gauges, the sensor comprises four gauges: two central gauges and two peripheral gauges. At the center of the diaphragm, the curve of stresses is "flat", that is to say that the zone of maximum stress is relatively "long". The two gauges disposed in this region are the I-shaped type, that is to say that they have a single part which has a substantial length. At the periphery of the diaphragm, on the contrary, the curve of stresses is very "pointed", that is to say that the zone of maximum stresses has a reduced length. This is why the gauges are placed in U-shapes which are each constituted by two half-gauges of reduced length (forming the limbs of the U) connected at one of their extremities by a conductive connection.
One of the problems in the implementation of these sensors resides in the fact that U- and I- shaped gauges have very different forms. Further, U- shaped gauges comprise two supplementary ohmic contact zones which introduce additional series resistance. It is therefore very difficult to give the four gauges identical resistances. Additionally, the resistance of the gauges themselves and the resistance of the metal-semiconductor contact (zone of ohmic contact) does not vary in the same way with temperature, which makes it impossible to balance the bridge at zero pressure for all temperatures in a given range of temperatures (offset effect). It should be added that if the electrical conductors which connect the gauges together do not all have the same electrical resistance, the effect of temperature variations on these conductors can also introduce drifts in the offset of the bridge. Offset compensation is generally achieved by external compensation elements, for example resistances, which do not have the same temperature coefficient as the gauges of the bridge. It is therefore more difficult to correct the offset in a satisfactory manner in a given range of temperatures as the initial offset to be compensated becomes larger.
Another difficulty resides in the fact that, to obtain an accurate measurement, it is necessary to protect the gauges with respect to electrostatic charges which can be produced on the surface of gauges of semiconductor material, or more precisely on the surface of the protective layer of the gauges. These charges have the effect, directly or indirectly, of modifying in a variable and random manner the cross section of the effective path for the current flowing in each gauge. In the case where the gauge is formed from doped polycrystalline silicon, the electrostatic charges can affect the carrier density in the gauge to a depth of up to 100 A. To eliminate this effect, there is formed on each gauge an electrostatic screen which thus avoids the formation of electrostatic charges. The formation of these screens and their connection to the rest of the circuit are delicate operations, and risk in their turn introducing into the measurement bridge formed by the four gauges asymmetries leading to an unbalance of the bridge and inducing drifts with temperature.
An object of the invention to provide a pressure sensor on an insulating support capable of operating at high temperature (for example 200.degree. C.), which has a bridge of piezoresistive semiconductor gauges having a more symmetrical geometry than bridges of the prior art, which permits the voltage unbalance of the bridge at zero pressure to be eliminated or at least very substantially reduced, so considerably simplifying the implementation of the compensation elements.
According to a first aspect of the invention disclosed herein and claimed in our parent application Ser. No. 07/479 889 now U.S. Pat. No. 5,081,437, the pressure sensor comprises an insulating support, four piezoresistive gauges formed on the insulating support in a semiconductor material, two gauges being U-shaped and two others being I-shaped, and is characterized in that each of the four gauges comprises two half-gauges, each half-gauge comprising an elongate sensing zone of semiconductor material and of reduced width in the plane of the support, two ohmic contact zones disposed at the ends of the half-gauge, and two connection zones in semiconductor material and of greater width disposed between said sensing zone and said ohmic contact zones, the form of the two connection zones being the same for the eight half gauges.
As a result of these characteristics, the global structure of the sensing element constituted by the four piezoresistive gauges is more symmetrical than in the embodiments of the prior art. Additionally, the particular disposition of the gauges permits the surface of the conducting parts serving to connect the gauges together and to the rest of the measurement bridge to be reduced.
Preferably, said sensor also comprises eight screen electrodes in conductive material, each screen electrode covering one of said half-gauges, one end of each screen electrode being electrically connected to a contact zone of the corresponding half-gauge.
A problem which can be encountered with semiconductor on insulator gauge sensors resides in the form of the piezoresistive gauges formed on the insulating support.
Thus, a component formed on an insulator typically comprises a flat insulating support, for example in silicon oxide, on which is disposed the component itself, which is of suitably doped semiconductor material. The component is defined by etching away an initial layer of the semiconductor material so as to leave in existence only the part of the layer necessary to make the component. The component therefore has the form of a "mesa" which stands proud of the upper surface of the insulating support.
In the case where the component is a piezoresistive element, the component ought to have the shape of an elongate bar of constant width. Often, however, the piezoresistive element is provided at each end of the bar with a rectangular electrical connection zone of which the width is greater than that of the bar, to minimize the dispersion of the resistivity characteristics in the manufacturing process.
To define the piezoresistive element and to connect it to the rest of the circuit of which it forms part, it is necessary to deposit on the element successive layers of various kinds, for example a passivation layer followed by a metallization layer followed by an insulation layer, etc. These different layers necessarily overflow or overlap the semiconductor layer defining the piezoresistive element. In a cross-sectional view, these overlapping layers may cause increased stress concentrations in a narrow region around the lateral corner edges of the piezoresistive layer. These stresses concentrations may cause in time a relaxation of the structure and result in some changes of resistivity of the piezoresistive material in that narrow region with corresponding uncontrolled offset variations.